Frequency shift telegraph communication



Feb.26,1946. C WHANSELL 2,395,478

FREQUENCY SHIFT TELEGRAPH COMMUNICATION lNvENToR l am Wfmll ATTO RN EYFeb. 2s, 1946. Wl HANSELL 2,395,478

FREQUENCY SHIFT TELEGRAPH COMMUNICATION Filed March 6, 1941 3Sheets-Sheet 2 #ab/,Ffreaaefccyy-@Zr fr' Noma @V Mw ig/0 I M /4 3oINVENTOR @armee W y BY ATTORNEY Feb; 26, 1946. C, W, HANSE'LL 2,395,478

FREQUENCY SHIFT TELECJRAPH COMMUNICATION Filed March 6, 1941 3Sheets-Sheet 3 IN'VENTOR .ATTORNEY Patented Feb. 26, 1946 FREQUENCYSHIFT TELEGRAPH COMMUNICATION Clarence W. Hansell, Port Jefferson, N.Y., assignor. to Radio Corporation of America, a corporation of DelawareApplication March 6, 1941, Serial No. 381,945

16 Claims.

This application relates to frequency modulation signalling systems andin particular spaced wave telegraphy systems such as disclosed in FinchPatentNo. 2,225,691, dated December 24, 1940, Hansell application SerialNo. 324,051, filed March 15, 1940, now U. S. Patent No. 2,293,501, datedAugust 18, 1942, and `Hansell application Serial No. 395,556, filed May28, 1941, now U. S. Patent No. 2,339,851, dated January 25, 1944.

In Vmy two applications I have described combinations of equipment forcarrying out radio telegraph communication by means of frequency shiftkeying at the transmitter. In this kind of frequency shift system thefrequency of an oscillation or other wave source is'keyed between twovalues one of which represents marks and the other of which representsspaces. In this mode of communication the transmitted power isapproximately equivalent to that which would be obtained `if twoordinary telegraph transmitters, on different frequencies, were employedto carry the same messages simultaneously by turning on the power of onetransmitter when the Yother is turned 01T and vice versa.

In my application Serial No. 324,051, led March 15, 1940, I have shownthat, if this type of communication is carried out with suitableequipment, a very worth while reduction in the elfects of signalelongations and distortions, due to presence of multiple signaling pathsof different time delay, may be obtained. To obtain full advantage ofthe system, for reducing multipath eects, the amount of frequencydifference between the two signaling waves should be greater than. thevalue of the highest required signaling frequency.

To obtain the greatest benefit from frequency shift signaling both fromthe standpoint of reducing multipath distortions and of reducing theeffects of noise, it is desirable that the amount of frequency shift bemade quite large in proportion to the highest equivalent modulationfrequencies required to reproduce the signals.

In long distance short wave radio communication the transmitters arefrequently used alternatively for either telegraph, facsimile ortelephone communication. Reasonably satisfactory telephony, or programtransmission, requires a modulation frequency band of about 6000 cyclesand a corresponding double side band space of 12,000 cycles. It has beencommon practice to space transmitters 15,000 cycles apart, which allowsa tolerance of plus or minus 1500 cycles from assigned frequenciesbefore `there can be an interfering overlapping offrequenciestransmitted by one transmitter into the frequency bandassigned to another transmitter.

Under these circumstances it is reasonable to assume that, when weemploy frequency shift telegraphy, we may permit frequency shifts whichare large' enough to occupy a total frequency band of up to 12,000cycles.

In radio telegraph practice a common future traic requirement maycorrespond approximately to an assumed keying rate of 28.5 letters ortime spaces per second, or 1710 letters or time spaces per minute, inseven unit printer code systems such as disclosed by J. B. Moore and R.E. Mathes in their United States Patent No. 2,183,147, dated December12, 1939. This would correspond to the use of six time divisionmultiplex channels each working at a rate of about 300 characters, orlive-letter words, per minute over the one radio circuit. This method ofmultiplexing is described in a paper by J. L. Callahan, R. E. Mathes andA. Kahn, entitled, Time division multiplex in radiotelegraphic practice,which appears in Proceedings of the Institute of Radio Engineers forJanuary, 1938.

For this signaling speed and number of channels the signal pulses occurat a rate of per second or, say, approximately 200 per second.

To reproduce these pulses requires a modulation rateA corresponding tocycles per second plus harmonics of 100 cycles up to and including, say,the fth harmonic, or 500 cycles per second. This modulation rate is muchless than that which would be required to occupy all of the availablefrequency band assigned to each transmitter.

In widening the frequency band in accordance with the inventionsdisclosed in my applications referred to, to obtain a greate reductionin multipath distortion and a greater suppression of the eifects ofnoise and interference it is necessary to keep in mind other phenomenamay be encountered which detract from the expected improvement.

For example, widening the frequency band occupied by the signalingcurrents or waves reduces the effects of noise by an amplitude ratioproportional to the increase in spacing between the two transmittercarrier frequencies only so long as the transmitter carrier currentreaching the frequency shift detector in the receiver re mainsconsiderably stronger than the peak value of the noise and'interference.In practice it may be stated that the initial interfering currentcomponents should not exceed `in combined instanintervals of time or thehigh gain in signal to noise ratio, due to increased frequency shift,begins to be reduced. When the width of the frequency band of thereceiver is increased, to accommodate a greater transmitter frequencyshift, the relativepeak value of significant interfering noise increasesby an amplitude ratio lying between the ratioof band widths and thesquare root of the ratio.. depending upon the nature of the noise.Consequently, although widening the band occupied usually results innoise suppression the percentage of time during which noise can wipe outthe signal is also inicreased.

In describing my invention reference will be made to the attacheddrawings wherein Figure l is a graph used to illustrate the character offrequency shift wave and the relative maximum amplitudes, and frequencyspacing, of the current components thereof in the frequency spectrum forthe conditions described in the example of six channels, time divisionmultiplex using seven unit code and about 60 words per minute speed foreach of the six' channels. Figure 2 is a diagrammatic showing of theessential units of a Y receiver arranged in accordance with my inventionto receive wave energy of the character of that shown in Figure 1 and toimprove the signal to noise ratio during such reception. Figures 3 and 4illustrate the receiver of Figure 2 with circuits added for reducing theeffects of unequal fading of signals in the two` transmitted frequencybands. Figure 5 illustrates a diversity receiving system employingreceivers of the type illustrated in Figures 2 and 3. Figure 6illustrates by block diagram a transmitter for producing andtransmitting carrier shifted keyed wave energy of the nature illustratedin Figure 1 of the drawings.

In examining Figure l a significant observation is that the componentfrequencies of frequency shift current required to reproduce thetelegraph signals are notv uniformly distributed throughout the bandoccupied but are grouped around the two transmitter carrier frequencies.There is a relatively large portion (space) in the middle of thefrequency band in which energy of signaling current components is verysmall but in which noise or interference may enter to wipe out thesignal at times.

Wave energy of the nature illustrated in Figure 1 may be produced andtransmitted by systems such as disclosed in Finch Patent No. 2,225,-691, dated December 24, 1940, and in Hansell U. S. application SerialNo. 324,051, filed March i5, 1940. A satisfactory means to accomplishthis may comprise an oscillation generator with means to shift itsfrequency an amount sufficient to prevent side band overlap as explainedin detail above in connection with Figure 1. In Figure 6 I have shown byblock diagram a transt mitter for doing this.

In accordance with this invention I also propose to suppress receivedcurrents in this unused frequency band, by means of frequencyselectivity, so as to suppress noise and interference currents at theinput to the frequency shift detector but, at the same time, to combinethe currents derived from the two signalling frequency bands into asingle chain of amplifier, amplitude limiter andvfrequency shift.detector circuits. One arrangement for doing this is illustrated inFigure 2.

In Figure 2 I have illustrated a receiver for frequency shift keyedsignals in which energy picked up from the transmitter by means of anantenna I0 is amplified and heterodyned down to a band of intermediatefrequencies. The means for accomplishing this may be conventional andmay comprise an amplifier and mixer and I. F. amplifier in unit I2excited by the wave energy received from I0 and by oscillations receivedfrom a source in unit I4. Energy at the intermediate frequencies isapplied to two band pass circuits IG and I8 arranged to pass only therequired frequency bands aroundV the two intermediate frequencysignaling carrier currents (Fig. l) and to suppress currents of allother frequencies. This reduction in the width of the band used forpassing the energy used for recording purposes reduces the total noisepower which reaches the frequency shift detector unit thereby improvingthe initial signal to noise ratio and making it` possible to obtain thenoise suppression inherent inwide band frequency modulation systems,when the carrier current isV higher than the noise, for a much largerproportion of time.

After the two bands of currents have passed` through the filters I6 andI8 they are combined again into one circuit and fed to the intermediatefrequency amplifier in 20. This amplifier also includes an amplitudelimiter. The amplified and limited wave energy of improved initialsignal to noise ratio is fed to a frequency shift detector 22, Thefrequency shift detector 22, and its output amplier, if one is used, aredesigned to respond to all required signaling frequencies which, ifnecessary, may include response down to zero frequency, corresponding todirect current signal output of either polarity. The' resulting outputcurrent, which usually will be a direct current which reverses polaritywhen the transmitter frequency is shifted from one value to the other,may be utilized to operate a signal recorder, printer mechanisms or anyother suitable terminal receiving equipment at 24. This output currentof reversed polarity may also be. considered as two currents utilizeddierentially,

The apparatus used in units I0, I2 and I4 may be conventional and it isbelievedunnecessary to describe and illustrate the same in detail here.The filters I6 and I8 may be band pass filters of any known type andhere again itis believed unnecessary to illustrate and describe detailsthereof. In some cases I may replace the band pass filters by high andlow pass filters with the cut-offs adjacent each other but separated bythe band of frequencies to be attenuated. I may also employ resistancenetwork terminations and coupling amplifier tubes at input and outputends of the filter, if desired, to minimize the effect of one filter onthe characteristics of the other.

The intermediate frequency amplifier may be of any approved type as maybe the amplitude limiter in unit 2|).A The same remarks apply to thefrequency modulation detector in unit 20 and the recorder in unit 24.

. Due to the action of the limiter ahead of the frequency shift detectorthe current delivered to' the detector will possess a center frequencyaround one transmitter frequency or the other depending upon whichtransmitter frequency component reaches the limiter with the greaterstrength. Transition from one frequency to theA other will be relativelyabrupt in the outputfrom frequency will be phase modulated by currentsat the other frequency, during the transient periods, while multipath ispresent, as explained in my United States application Serial No.324,051, led

other transmissions, preferably frequency shiftV transmissions of asimilar kind. There may be one, two, or more, working bands for othertransmitters between the two bands occupied by the first transmitter.Thus the frequency shift of each transmitter may be much greater thanthat required to double its frequency band but `the frequency space itoccupies is only twice that required by the ordinary power on-off keyedtransmitter.

Although the system requires twice the frequency band for eachtransmitter which would be occupied by the older type transmitters it iscapable of operation at much higher average speeds because of thereduction in effects of multipath and noise. Consequently, wheneverything is taken into account it provides more maximum trafficcapacity for a given total frequency band than the older systems and, atthe same time, improves the reliability with which traiiic at anyparticular speed or number of channels can be maintained.

A further consideration in applying wide frequency shifts is that theremay be fading n the two transmitter frequencies which are not alike.This might cause signal unbalances, or even complete drop-outs, when thelength of each dot becomes less than the time delay of secondaryionosphere paths. To reduce this possibility I may employ specialcircuits at the receiver, such as illustrated in Figures 3 and 4, anddiversity re ception as illustrated in Figure 5.

In the receiver of Figure 2,` when no signal current is being received,substantially equal amounts of noise power will pass through the twoband pass filters which tend to balance out in the frequency modulationdetector. Therefore, within the used output current frequency band,absence of input signal results in substan-v tially no receiver outputcurrent. As a consequence, during frequency shift signaling, if bothsignaling currents fade down below the noise level at the receiverinput, the receiver output tends to disappear.

On the other hand, if due to selective fading only one of the signalingcurrents disappears then the receiver continues to provide a unipolarkeyed output due to the remaining keyed current. 'Ihis unipolar outputmay be sucient to maintain traffic in many cases. To utilize it in areverse current system it is desirable to provide means to derivereverse current signals from the unipolar current signals.

One means for doing this is illustrated in the connections between thedetector and recorder elements in Figure 3. .According to `this schemethe signaling current passes through electrical condensers 30 ofdielectric capacity large enough to properly pass the signallingcurrents at normal'minimum and higher keying speeds. Any unbalance inthe two output unidirectional currents of different polarity which lastsfor a relatively long time, such as unbalances caused by selectivefading, is reduced or stopped. by the condensers.

By paralleling each condenser with an adjustable resistance 34 theeffect of the condensers 30 in suppressing unbalances can be adjustedand, at the same time, the condensers may be given any desired timeconstant of response to increasing or decreasing unbalances in theoutput currents of different polarity.

The arrangement of Figure 4 is anotherar-v rangement for accomplishingthe same results as Figure 3. In this case a coupling transformer 40 isused to pass on the normal speed signaling currents and adjustableresistances 3d again serve to adjust the time constant of currentresponse to unbalances caused by selective fading. The transformer ofFigure 4 may also be replaced by'a simple inductive reactance bridgedacross the output leads from the frequency shift detector in which caseresistance in series with the reactance may be adjustable to control thetime constant of response to selective fading.

output signals with no change in the average or direct current flowingin the utilization circuit. Therefore, the coupling condensers, ortransformer prevent any direct current component of final output currentwhich could disturb operation of the following signal handling andrecording equipment. If the receiver output is delivered to a recorder,it is undesirable to allow any D. C. component of current which mightdeect the recording element against its mechanical stop in a manner tomake the signals less intelligible. The recorders usually do have amechanical stop to cause formation of more nearly rectangular waverecords, in spite of noise, as an aid to intelligibility of the record.If no stops were used then the presence of a D. C. current, due tocarriercurrent unbalances, would be somewhat less important but then therecord slip would need to be wider or the response to signals would needto be reduced, and intelligibility would be lessened.

If, instead of signal recorders, the signals were fed into printerequipment similar difficulties would be encountered due to the need toprevent biasing of the associated relays and other equipment.

On the other hand, taking signals through condensers or transformersexclusively tends to pre-A vent response to Very low signaling speeds,frequently required, or to steady marking or spacing conditions.Therefore, I provide the adjustable shunts around the condensers andtransformers to permit adjustment according to conditions of service.

In Figure 5 I have shown a space diversity receivingl system. Thissystem makes use of the theory and principle involved in the diversitysystems invented by H. H. Beverage and H. O.

Peterson and disclosedin their patents, such as for example: H. H.Beverage et al. Patent #1,- 819,589, dated August 18, 1931; H.H.,Beverage et al.. Patent #1,874,866, dated August 30, 1932;H..H.'Beverage et al. Patent #1,987,889, dated January 15, 1935; H. O.Peterson Patent #1,863,- 695dated June 21, 1932. A similar system hasalso.- been disclosed in my Patent #1,803,504 to which the features ofthe present invention have been added. i

In. the arrangementof Figure 5 there are indicated three spaced.receiving antennas and three receivers, like the receiver of Figure 3,for receiving the same frequency shift keyed transmitted signals.Outputs from the three receivers: are combined into one circuit toprovide a combined signal of better quality and reliability than can beprovided from a single receiver. The combined signal'is then utilized tocontrol a carriercurrent keyer for sending signals. over a wire line.-to a central trafhc office where they may be recorded by standardprinting telegraph instruments, or ink recorders when manualtranscription is employed.

In some cases in association with the present invention I may alsoprefer to employ frequency diversity to overcome effects of fading,which I may do by phase, frequency or amplitude modu- `lating the twotransmitted carrier currents at a frequency higher than the highestrequired component modulating frequency required to satisfactorilyreproduce the transmitted signals. The

principles upon which this kind of diversity are based are disclosed inGoldsmith U. S. Patent No. 1,821,383 and in my application Serial No.324,- 052, filed March 15, 1940, now U. S. Patent No. 2,278,77 9, datedApril 7, 1942. I may also add to the diversity receiving system theprinciples and arrangements disclosed in my application No. 326,12'9,filed May 27, 1940, now U. S. Patent No. 2,249,425, dated July 15, 1941,for making the receiver, or receivers, with strongest signal inputdeliver greater fractions of the total signal output'. Y

In all of the arrangements of Figures 2 to 5, inclusive, and in otherarrangements like them, I contemplate employing automatic tuning controlof the Ytype described in my application No. 395,556, led May 28, 1941.

I claim:

1. In a diversity receiving system for wave energy comprising aplurality of keyed carrier currents separated in the frequency spectrumby an 1 amount greater than that required to prevent overlapping ofnecessary side band currents of eachcarrier current in the samefrequency band at the highest keying frequency, comprising a pluralityof receivers each comprising means for rejecting frequencies lyingbetween the bands occupied by the carrier currents as shifted and theirnecessary side bands, and a frequency shift modulation detectorresponsive only to all the necessary frequency shift modulationcomponents of the signalling currents, and means for combining the.outputs of all of said detectors.

2. The method of communication by frequency shift keyed carrier currentswith reduction of noise and multipath distortion including the followingsteps, producing a substantially fixed frequency shift of the keyedcarrier from one position in the frequency spectrum to another positionin the frequency spectrum separated from said first position by a bandof frequencies greater than the sum of half the necessary side bands ofsaid carrier' in each of the positions to which the carrier isi shiftedat the maximum rate at whichit is shifted, transmitting the WaveV energyso keyedV and shifted, receiving the transmitted wave. energy, producingcurrents representing the .i received ienergy, amplifying kthe producedcurrents While rejecting currents of frequencies lying between the bandsof produced currents representing Ythe necessary side bands of thecarrierv in itsshifted positions', passing the currents not rejectedthroughla common path and subjecting the currents passed by the 'commonpath to frequency shift demodulation. l

' 3..' The method of communication by frequency shift keyed carriercurrents with reduction of noise and multipath distortion including thefollowing steps, producing a substantially fixed frequency shift of thekeyed carrier from one posi-,- tion in the frequency spectrum to anotherposition in the frequency spectrum separated from said rst position by aband of frequencies. greater than the sum of half the necessary sidebands of said carrier in each of the positions to which the carrier isshifted at the maximum rate at which it is shifted, transmitting thewave energy so keyed and shifted, receiving the transmitted wave energy,producing currents representing the received wave energy, amplifying theproduced currents while rejecting currents of frequencies lying betweenthe bands of produced currents representing the necessary side bands ofthe carrier in its shifted positions, limiting the amplitude of thecurrents not rejected, and subjecting the limited currents to frequencyshift demodulation to derive direct current which reverses polarity asthe carrier shifts from one position to another position.

4. The method ofcommunication by frequency shift keyed carrier currentswith reduction ofr noise and multipath distortion including thefollowing steps, at the transmitter producing a substantially fixedfrequency shift of the keyed carrier from one position in the frequencyspectrum to another position in the frequency spectrum separated fromsaid rst position by a band of frequencies greater than that required toprevent overlapping-of necessary side bands in each of the positions towhich the carrier is shifted at the maximum rate at which it is shifted,transmitting the wave energy so keyed and shifted, at the receiverproducing currents representing the transmitted wave energy, amplifyingthe produced currents while rejecting currents of frequencies lyingbetween the bands of currentsV representing the necessary side bands ofthe carrier inr its shifted positions, translating currents not rejectedin a common path, limiting the amplitude of the translated currents andsubjecting the translated currents to frequencyshift demodulation.

5. In a communication system using a transmitted carrier wave shifted infrequency by signals from one substantially fixed frequency to a secondsubstantially fixed frequency, separated from said-first frequency by anamount such that the wave in its different positions may be subjected toselective fading, an amplifier for picking up said wave and derivingtherefrom currents correspondingly shifted in frequency, filters coupledwith said amplifier for passing the currents corresponding to thecarrier and its necessary side bands in said two positions, a frequencyshift detector coupled to said filters for demodulating the currentspassed by said filters to derive therefrom direct currents whichreversepolarity as the carrier is shifted from one position to the.

other, said demodulator having an output wherein said direct currentsare differentially utilized and are, in effect, bi-polar currents, andmeans in said output for preventing uni-polar currents from beingproduced in the event of selective fading which reduces the value of thecarrier in one of its positions.

6. rIn a communication system using `a transmitted carrier wave shiftedin frequency by signals from one substantially fixed frequency to asecond substantially fixed frequency, separated from said firstfrequency by an amount such that the wave in its different positions maybe subjected to selective fading, an amplifier for picking up said waveand deriving therefrom currents correspondingly shifted in frequency,filters coupled with said amplier for passing the currents correspondingto the carrier and its necessary side bands in said two positions, acurrent amplitude limiter coupled to said filters, a frequency shiftdetector coupled to said limiter for demodulating the currents passed bysaid limiter to derive therefrom direct currents which reverse polarityas the carrier is shifted from one position tothe other, saiddemodulator having an output wherein said direct currents appear, whichcurrents are, in effect,` bi-polar currents, a recorder and connectionscoupling said output of said demodulator to said recorder and reactancein said connections for preventing uni-polar currents from beingproduced and supplied to said recorder in the event of selective fading,which reduces the value of the carrier in one of its positions.

'7. In a communication system, means for producing keyed carriercurrents shifted through a frequency range greater than that required toprevent overlapping of necessary side band currents of each carriercurrent in the same frequency bands, circuits for transmitting saidcarrier currents, a receiver energized by said transmitted currents,filters at said receiver for rejecting frequencies lying between thebands occupied bythe carrier currents and their necessary side bands, arecorder, a frequency shift detector coupled to said filters fordemodulating the currents passed by said filters7 said detector havingan output wherein is produced direct current the polarity of whichreverses as the carrier currents are shifted in frequency, connectionsfor impressing said direct current on the said recorder, and reactancesin said connections between said recorder and said output formaintaining substantially equal said direct currents of opposed polarityin the presence of selective fading of the carrier in the range throughwhich it is shifted.

8. A communication system as recited in claim 6 wherein said reactanceis a transformer having a primary winding coupled to the demodulatoroutput and having a secondary winding coupled to the recorder.

9. A system as recited in claim 6 wherein said reactance is atransformer havingv a primary winding coupled to the demodulatbr outputand a secondry winding coupled to the recorder and wherein a resistanceconnects one end of the primary winding to one end of the secondarywinding and a second resistance connects the other end of the primarywinding to` the other end of the secondary winding.

10. A system as recited in claim 6 wherein said reactance comprises acondenser in each connection between the demodulator output and saidrecorder.

11. A system as recited in claim 6 wherein said reactance comprises acondenser in each connecel tion between the demodulator `output andtherecorder and wherein a resistance is connected in shunt to eachcondenser.

12. The method of communication by frequency shift keyed carriercurrents with reduction of noise and multipath distortion includingthese steps, at the transmitter producinga carrier and shifting thefrequency of the carrier in accordance with signals between twofrequencies spaced from each other by a frequency band greater than thatrequired to prevent overlapping of necessary side band currents, of the,car-i rier current in each ofV its positions, which band of frequenciesis at least several hundred cycles wide, transmitting the wave energy soshifted in accordance with signals, at the receiver rejecting currentsof frequencies lying between the bands occupied by the carrier currentin its shifted positi-ons and the necessary carrier side bands, limitingthe amplitude of the remaining shifted carrier and side bands anddetecting the said shifted carrier to recover the signals.

13. The method of communication by frequency shift of carrier currents,keyed in accordance with signals, with reduction of noise and multi-pathdistortions including these steps, at the transmitter producing afrequency shift of the keyed carrier currents from a marking frequencyto a spacing frequency separated from the marking frequency by a band offrequencies substantially greater than that required to preventoverlapping of necessary side band currents of each carrier current inthe same frequency band at the maximum keying rate, transmitting thewave energy so shifted, at the receiver receiving the transmitted waveenergy,`and selectively separating the marking frequency and itsnecessary side bands from the spacing frequency and its necessary sidebandsfor recording purposes while rejecting frequencies lying betweenthe bands occupied by the spacing and markingfrequency currents andtheir necessary side bands.

14. The method of communication by frequency shift keyed carriercurrents with reduction of noise and multi-path distortions includingthese steps, producing a frequency shift of they keyed carrier currentfrom a rst frequency to a second frequency separated from said rstfrequency by a band of frequencies substantially greater than thatrequired to prevent overlapping, in the same frequency band, ofnecessary side band currents of each carrier current at the maximumkeying rate, transmitting the wave energy so shifted, receiving thetransmitted wave energy, rejecting currents of frequencies lying betweenthe bands occupied by the carrier currents and their necessary sidebands, passing the two carriers and their side bands only through acommon path, limiting the amplitudes of the currents passed by saidcommon path and demodulating and recording the limited currents.

15. In a communication system, means for producing wave energy ofcarrier frequency and shifting the said wave energy in accordance withsignals from a first frequency to a second frequency spaced from thefirst frequency by an amount greater than that required to preventoverlapping of necessary side band currents of the said carrier in itsshifted positions at the highest signal frequency, a receiver for saidcurrents comprising selective circuits which reject frequencies lyingbetween the bands occupied by thescarrier current in its shiftedpositions and its necessary side bands, meanszfor passing saidcurrentsAthrough said selective circuits, means for combining the currentspassed by saidselective circuits, an amplitude limiter connected withsaid last named means for removing amplitude modulation from thecombined currents, .and frequency shift detecting means coupled withsaid limiter.

.16. The method rof communication by frequency shift keyed carrierlcurrents With reduction of `noise and multi-path distortion includingthe steps of producing wave energy of carrier wave `frequency shifted infrequency .in Aaccord-- ancewth signals ,from a first frequency to asec- Asion characteristics for the carrier inlitsshifted positions maybe different, which band is at least several hundred cycles Wide so thenecessary` carrier side bands occupy a space considerably less than thespace between said first and second frequencies, transmitting saidcarrier Wave energy and necessary side bands, receiving and ampli= fyingthe carrier wave energy and its necessary side bands while rejectingenergy at frequencies lying between adjacent side band frequencies,

and rectifying the carrier and side band frequencies not rejected.

CLARENCE W. HANSELL.

